1. Field of the Invention
Aspects of the present invention generally relate to a printer, a sheet feeding apparatus, and an image forming apparatus equipped with the sheet feeding apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer or a copying machine is equipped with a sheet feeding apparatus for feeding a sheet to an image forming unit.
Such a sheet feeding apparatus is provided with a separation mechanism for separating and feeding sheets one by one.
Japanese Patent Application Laid-Open No. 10-72142 discusses a sheet feeding apparatus equipped, as a separation mechanism, with a feeding roller configured to feed the uppermost sheet of sheets stacked on a feeding tray, and a separation slope with which the sheet fed by the feeding roller comes into contact. In the sheet feeding apparatus discussed in Japanese Patent Application Laid-Open No. 10-72142, sheets are fed by the feeding roller, and solely the uppermost sheet is bent to move over the separation slope, whereby the uppermost sheet is separated from the second sheet and subsequent sheets.
However, in the sheet feeding apparatus discussed in Japanese Patent Application Laid-Open No. 10-72142, the width of the separation slope that can contact the sheet (i.e., the area of the separation slope with the sheet thickness included) is constant, so that the apparatus leaves room for improvement in terms of separating and feeding of various kinds of sheets.
In the following, the above sheet feeding apparatus will be described in detail.
In the separation mechanism for separating sheets from each other by using the separation slope and the feeding roller, it is necessary to bent the uppermost sheet of sheets stacked on the feeding tray to thereby separate the uppermost sheet from the sheet directly under the uppermost sheet.
For the sheet to move over the separation slope, it is necessary to apply, to the sheet, a larger force than the force for maintaining a sheet plane with the rigidity of the sheet, i.e., a larger force than the force required for bending the sheet to move over the separation slope (hereinafter referred to as a bending force). The separation mechanism configured to separate sheets from each other by using the separation slope and the feeding roller, the sheet being fed by the feeding roller is bent by coming into contact with the separation slope and receiving a reaction force from the separation slope.
An examination by the present inventor has made it clear that, when the sheet is bent by the reaction force from the separation slope, the sheet is bent more easily when the reaction force per unit area received from the separation slope (hereinafter referred to as a face pressure) is larger than the bending force of the sheet.
Here, the bending force of the sheet varies depending on the grammage, rigidity, etc., of the sheet. For example, in the case of a sheet having high grammage and rigidity (thick paper sheet or the like), the bending force becomes large, whereas, in the case of a sheet having low grammage and rigidity (thin paper sheet or the like), the bending force becomes small.
On the other hand, the reaction force the sheet being in contact with the separation slope receives from the separation slope varies depending on the conveyance force by the feeding roller. However, if the conveyance force by the feeding roller is constant, the face pressure the sheet receives from the separation slope varies depending on the contact width (area) in which the sheet is in contact with the separation slope. More specifically, when the width in which the separation slope and the sheet are in contact with each other is set to be large, the face pressure becomes low, whereas, when the width in which the separation slope is in contact with the sheet is set to be small, the face pressure becomes high.
Thus, by setting the width in which the separation slope is in contact with the sheet to be small to increase the face pressure, the sheet is easily bent when the sheet having a large bending force is fed. Here, due to the friction force between the uppermost sheet and the sheet directly under the uppermost sheet, there is exerted a moving force in the feeding direction between the uppermost sheet and the sheet directly below the uppermost sheet (i.e., the second sheet). Thus, if the face pressure is increased, in the case where a sheet having a small bending force is fed, the second sheet may be simultaneously fed due to the frictional force. This may results in double feeding with two sheets being fed in an overlapped state.
On the other hand, if the width in which the separation slope and the sheet are in contact with each other is set to be large to diminish the face pressure, even when the sheet having a small bending force is fed, the above-mentioned double feeding is made unlikely to occur. However, if the face pressure is diminished, the reaction force (face pressure) the sheet receives from the separation slope becomes rather small when a sheet having a large bending force is fed, so that, in some cases, the sheet cannot be bent to be separated and fed.
As described above, in the sheet feeding apparatus discussed in Japanese Patent Application Laid-Open No. 10-72142, the width in which the separation slope is in contact with the sheet is constant, so that, if the kind of sheet (the grammage, rigidity, etc., of sheet) is changed, double feeding may occur, and thus, in some cases, sheets cannot be separated and fed.